netstandard 2.0 library, with a .net 4.6.1 console app as a starter: works

When the symbolics nuget is added to each of the projects (named before), nuget shows a constant warning: warning NU1701: Package ‘MathNet.Symbolics 0.15.0’ was restored using ‘.NETFramework,Version=v4.6.1’ instead the project target framework ‘.NETStandard,Version=v2.0’. This may cause compatibility problems.

That basically says: as long as your dll only uses netstandard20 compatible classes, you are fine. If not, an exception is thrown. Feels kind of itchy. But might work.